IPv6 路由器报告超大报文，由源端决定怎样适应路径

Wed, 06 Oct 2027 00:00:00 +0000

IPv6 路由器在向目的地转发报文时，不会替它进行分片。如果报文大于下一条链路所能承载的大小，路由器通常丢弃它，并发送 ICMPv6 Packet Too Big 反馈。源端随后可以调整报文大小，或在适当情况下通过 IPv6 分片扩展头发送分片；这些分片在目的端重组。明确各阶段由谁负责，是判断超大报文故障的基础。

坏掉的样子

假定路由器会自动拆开过大的 IPv6 包，会掩盖与大小相关的故障。相同地址与路由上，小 echo 可以成功，大 echo 却失败。端点本地接口可能支持原始大小，中间链路却不支持。应检查整条路径约束，不能仅因接口配置了 MTU 1500，就推断该大小能够端到端通过，也不能用一次小探测成功替代完整大小检查。

Packet Too Big 反馈缺失，会使经典发现更难学到这个限制。分片被过滤，或者其中一片丢失，也可能阻止正确重组。能够生成分片扩展头，不证明沿途策略都允许相应报文。不能把源端分片当作正确路径大小与必要控制流量的万能替代，它有自己的交付条件，需要通过对应流量验证，而不是只检查发送选项已经开启。

反方向也有自己的路径估计。请求发送端学到 MTU，不会自动教会响应端怎样发送较大回复。因此双向 echo 测试需要分别了解两个方向。TCP 分段还不同于 IP 分片：TCP 发送端学到限制后，可以构造更小的段，而不必把每个 IP 包再分片。应先确定观察的是哪一层的切分，避免看到多个包就统称为分片。

快速验证

将脚本保存为 check-ipv6-fragmentation.py，准备 Docker 后运行 python3 check-ipv6-fragmentation.py。此前实验使用 Python 3.11.16、iproute2 6.1.0、iputils ping 和 tcpdump 4.99.3，运行于 Linux 6.12.76；本轮未重跑网络实验，以下为保留的历史观察。NET_ADMIN 与 SYS_ADMIN 支持四个嵌套命名空间。两台路由器和两个端点使用新建 veth，不修改宿主接口或路由：

from pathlib import Path
from tempfile import TemporaryDirectory
import subprocess,json,uuid
recipe='FROM python@sha256:528257d48c1da0dcecc2e725d1ae34498d60c965f1241e39cd6a85a8859bdf84\nRUN apt-get update && apt-get install -y --no-install-recommends iproute2=6.1.0-3 iputils-ping=3:20221126-1+deb12u1 nftables=1.0.6-2+deb12u2 tcpdump=4.99.3-1 traceroute=1:2.1.2-1 procps=2:4.0.2-3 && rm -rf /var/lib/apt/lists/*\nCMD ["python3"]\n'
program=r"""
from contextlib import ExitStack,contextmanager
from pathlib import Path
from tempfile import TemporaryDirectory
import json,os,select,signal,subprocess,time
def run(*args,check=True,input=None,timeout=8):
 result=subprocess.run(args,input=input,capture_output=True,text=True,timeout=timeout)
 if check:assert result.returncode==0,(args,result.returncode,result.stdout,result.stderr)
 return result
def ns(name,*args,**kwargs):return run('ip','netns','exec',name,*args,**kwargs)
def ip(name,*args,**kwargs):return ns(name,'ip',*args,**kwargs)
def stop(process):
 if process.poll() is None:
 process.terminate()
 try:process.wait(timeout=2)
 except subprocess.TimeoutExpired:process.kill();process.wait(timeout=2)
def spawn(stack,command):
 process=subprocess.Popen(command,stdin=subprocess.PIPE,stdout=subprocess.PIPE,stderr=subprocess.PIPE,text=True)
 stack.callback(stop,process);return process
def line(process,timeout=3):
 assert select.select([process.stdout],[],[],timeout)[0],'child response deadline exceeded'
 value=process.stdout.readline();assert value,process.stderr.read();return value
@contextmanager
def lab():
 with TemporaryDirectory(prefix='mtu-path-') as directory,ExitStack() as stack:
 root=Path(directory);proc=root/'proc';proc.mkdir();run('mount','-t','proc','proc',str(proc));stack.callback(run,'umount',str(proc))
 for name in ['client','r1','r2','target']:
 run('ip','netns','add',name);stack.callback(run,'ip','netns','delete',name,check=False);ip(name,'link','set','lo','up')
 def setting(name,key,value):
 assert name in ['client','r1','r2','target']
 path=proc/'sys'/key.replace('.','/')
 ns(name,'python3','-c',f'from pathlib import Path; p=Path({str(path)!r});p.write_text({str(value)!r});assert p.read_text().strip()=={str(value)!r}')
 yield root,stack,setting
def topology(setting,ipv6=False):
 links=[('client','c0','r1','a0',1,2,1,1500),('r1','a1','r2','b0',2,1,2,1280),('r2','b1','target','t0',3,1,2,1500)]
 for left,left_if,right,right_if,network,left_host,right_host,mtu in links:
 run('ip','link','add',left_if,'type','veth','peer','name',right_if)
 for name,interface,host in [(left,left_if,left_host),(right,right_if,right_host)]:
 run('ip','link','set',interface,'netns',name);ip(name,'link','set',interface,'mtu',str(mtu))
 address=f'2001:db8:{network}::{host}/64' if ipv6 else f'10.210.{network}.{host}/30'
 ip(name,'addr','add',address,'dev',interface,*(['nodad'] if ipv6 else []));ip(name,'link','set',interface,'up')
 family='-6' if ipv6 else '-4'
 for name in ['r1','r2']:setting(name,'net.ipv6.conf.all.forwarding' if ipv6 else 'net.ipv4.ip_forward',1)
 routes=[('client','default','2001:db8:1::1' if ipv6 else '10.210.1.1'),('target','default','2001:db8:3::1' if ipv6 else '10.210.3.1'),
 ('r1','2001:db8:3::/64' if ipv6 else '10.210.3.0/30','2001:db8:2::2' if ipv6 else '10.210.2.2'),
 ('r2','2001:db8:1::/64' if ipv6 else '10.210.1.0/30','2001:db8:2::1' if ipv6 else '10.210.2.1')]
 for name,destination,gateway in routes:ip(name,family,'route','add',destination,'via',gateway)
 if ipv6:
 deadline=time.monotonic()+4
 while any('tentative' in ip(name,'-6','address','show').stdout for name in ['client','r1','r2','target']):
 assert time.monotonic()<deadline,'IPv6 link-local DAD did not finish';time.sleep(.02)
def nft(name,script):return ns(name,'nft','-f','-',input=script)
def counters(name):
 result={}
 for item in json.loads(ns(name,'nft','-j','list','ruleset').stdout)['nftables']:
 rule=item.get('rule',{})
 if 'comment' in rule:
 for expression in rule['expr']:
 if 'counter' in expression:result[rule['comment']]=expression['counter']['packets']
 return result
def capture(stack,root,name,interface,expression,label):
 path=root/(label+'.pcap')
 process=subprocess.Popen(['ip','netns','exec',name,'tcpdump','--immediate-mode','-Z','root','-U','-n','-i',interface,'-w',str(path),expression],stdout=subprocess.DEVNULL,stderr=subprocess.PIPE,text=True)
 stack.callback(stop,process);deadline=time.monotonic()+3
 while time.monotonic()<deadline:
 assert process.poll() is None,'capture exited'
 if select.select([process.stderr],[],[],.1)[0] and 'listening on' in process.stderr.readline():return process,path
 raise AssertionError('capture did not become ready')
def captured(process,path):
 process.send_signal(signal.SIGINT);_,diagnostic=process.communicate(timeout=3);assert process.returncode==0,diagnostic
 return run('tcpdump','-nn','-vv','-r',str(path)).stdout

with lab() as (root,stack,setting):
 topology(setting,ipv6=True);target='2001:db8:3::2'
 small=ns('client','ping','-6','-n','-c','1','-W','1','-s','32',target);assert small.returncode==0
 tap,path=capture(stack,root,'client','c0','ip6','too-big')
 large=ns('client','ping','-6','-n','-M','do','-s','1400','-c','1','-W','1',target,check=False)
 feedback=captured(tap,path);assert large.returncode!=0 and 'too big' in feedback.lower(),(large.stdout,large.stderr,feedback)
 route=ip('client','-6','route','get',target).stdout;assert 'mtu 1280' in route,route
 reverse=ns('target','ping','-6','-n','-M','do','-s','1400','-c','1','-W','1','2001:db8:1::2',check=False)
 reverse_route=ip('target','-6','route','get','2001:db8:1::2').stdout
 assert reverse.returncode!=0 and 'mtu 1280' in reverse_route,(reverse.stdout,reverse.stderr,reverse_route)
 tap,path=capture(stack,root,'client','c0','ip6 and src 2001:db8:1::2','source-fragments')
 fragmented=ns('client','ping','-6','-n','-M','dont','-s','2000','-c','1','-W','2',target,check=False)
 trace=captured(tap,path);assert fragmented.returncode==0,(fragmented.stdout,fragmented.stderr,trace)
 assert 'frag (' in trace and trace.count('frag (')>=2,trace
 refused=ns('client','ping','-6','-n','-M','do','-s','2000','-c','1','-W','1',target,check=False)
 assert refused.returncode!=0 and 'too long' in (refused.stdout+refused.stderr).lower(),(refused.stdout,refused.stderr)
 print(json.dumps({'small_echo':small.stdout,'oversize_before_learning':{'stdout':large.stdout,'stderr':large.stderr,'returncode':large.returncode},
 'packet_too_big_capture':feedback,'learned_route':route,'reverse_path_learning':reverse.stdout+reverse.stderr,'reverse_route':reverse_route,'source_fragmented_echo':fragmented.stdout,
 'source_side_fragment_capture':trace,'dont_fragment_after_learning':{'returncode':refused.returncode,'stdout':refused.stdout,'stderr':refused.stderr}}))
"""
name='xbcatm-mtupath-'+uuid.uuid4().hex[:10]
def docker(*args,**kwargs):return subprocess.run(['docker',*args],capture_output=True,text=True,timeout=300,**kwargs)
try:
 with TemporaryDirectory(prefix=name+'-') as directory:
 root=Path(directory);(root/'Dockerfile').write_text(recipe)
 build=docker('build','-t',name,str(root));assert build.returncode==0,build.stderr
 result=docker('run','--rm','-i','--name',name,'--network','none','--cap-add','NET_ADMIN','--cap-add','SYS_ADMIN','--memory','256m','--pids-limit','128',name,'python3','-',input=program)
 assert result.returncode==0,result.stderr
 print(json.dumps(json.loads(result.stdout),sort_keys=True))
finally:
 docker('rm','-f',name);docker('image','rm',name)

端点链路 MTU 为 1500，中间路由器链路为 1280。静态全局地址使用 nodad，设置过程同时明确等待自动生成的链路本地地址离开 tentative 状态。这个就绪检查避免把最初地址设置延迟误判成转发或 MTU 故障。实验没有声称验证重复地址检测算法本身，只是把网络准备完成作为后续比较的前置条件。

小 echo 往返首先成功。随后使用 do 策略发送 1400 字节 echo 载荷，加上基础 IPv6 与 ICMPv6 头后，形成 1448 字节报文。它适合源端接口，却超过中间链路。捕获显示原始请求和路由器返回的 Packet Too Big，报告 MTU 1280；客户端路由查询随后记录该值。这样能区分源端本来就拒绝发送，与中间路径发现更小限制。

目标向客户端发送对应的超大探测，也为反向路径学到 MTU 1280。这一步明确准备较大 echo 回复，不假定一个端点的缓存同时覆盖两个方向。两个失败发现探测都保留在输出中，包含路由器反馈和路由状态，让后续成功所依赖的设置可见。请求方向已经正确，不代表响应方向无需独立检查。

客户端接着用 dont 策略发送 2000 字节 echo 载荷，允许根据已知上限在源端分片。抓包位于客户端接口，并限定客户端源地址，在两台路由器处理之前，就看到了标识相同的两个分片。工具显示数据偏移为零和 1232 字节，分片数据长度合计 2008，包含 ICMPv6 头。这直接证明分片已由发送端产生，而非后来由路由器拆开。

目的端重组并完成大小正确的返回交换后，echo 成功。用 do 策略重复相同载荷，则得到本地 message-too-long 错误与 MTU 1280。结果区分了路由器报告此前未知瓶颈、发现后源端主动分片，以及已知较小上限后的本地拒绝。这些是不同失败和交付阶段，不应全都写成同一种“包太大所以网络不通”。

来源

RFC 8200 IPv6，源端分片、分片扩展头与目的端重组，已核验 2026-10-15。
RFC 8201 IPv6 Path MTU Discovery，Packet Too Big 反馈与各方向路径估计，已核验 2026-10-15。
Linux ping(8)，载荷大小与 do/dont 路径 MTU 策略，已核验 2026-10-15。

分包 · XB 现场笔记

IPv6 路由器报告超大报文，由源端决定怎样适应路径

坏掉的样子

快速验证

推荐分片之前，先诊断大小与方向

来源